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Re: Mutation Observers: a replacement for DOM Mutation Events

From: Olli Pettay <Olli.Pettay@helsinki.fi>
Date: Wed, 12 Oct 2011 16:34:00 +0300
Message-ID: <4E959748.7060509@helsinki.fi>
To: Sean Hogan <shogun70@westnet.com.au>
CC: "Tab Atkins Jr." <jackalmage@gmail.com>, Adam Klein <adamk@chromium.org>, public-webapps@w3.org, Ojan Vafai <ojan@chromium.org>, rafaelw@chromium.org, rniwa@chromium.org, Jonas Sicking <jonas@sicking.cc>, annevk@opera.com, arv@chromium.org
On 10/12/2011 02:00 PM, Sean Hogan wrote:
> On 12/10/11 3:26 AM, Tab Atkins Jr. wrote:
>> On Mon, Oct 10, 2011 at 7:51 PM, Sean Hogan<shogun70@westnet.com.au>
>> wrote:
>>> On 24/09/11 7:16 AM, Adam Klein wrote:
>>>> - Is free of the faults of the existing Mutation Events mechanism
>>>> (enumerated in detail here:
>>>> http://lists.w3.org/Archives/Public/public-webapps/2011JulSep/0779.html)
>>> A simpler solution that is free from the faults listed in that email
>>> would
>>> be to have (at max) one mutation observer for the whole page context. I
>>> guess this would be called at the end of the task or immediately
>>> before page
>>> reflows.
>>> If a js lib (or multiple libs) want to provide finer grained mutation
>>> handling then let them work out the details.
>> That seems unworkably restrictive. It's very easy to imagine multiple
>> libraries listening for different kinds of things at the same time.
>> Libraries would just end up re-implementing event distribution, which
>> is something we can avoid by doing it correctly now.
> This proposal doesn't entirely avoid the issue of event distribution.
> There is no equivalent of event.stopPropagation() and hence no way to
> prevent mutation records being delivered to observers. The observers may
> have to be written with this is in mind.
> For example, what if two observers can potentially handle the same
> mutation - which one should handle it?
Both. Or the observers need to somehow communicate with each others to 
decide who handles it. This is no different to event listeners.
Event listeners don't know if there are other listeners before them or 
after them. You can have several listeners in the same target and 
different script libraries may have added them without knowing about 
each others.

> Alternatively, some code might respond to an attribute by adding content
> to the DOM. What if there are mutation listeners that could respond to
> that added content? Is it desired that they ignore or handle it?
> Another pattern that doesn't seem to be reliably handled is mutations
> within DOM fragments that are temporarily removed from the document.
> That is:
> - if the fragment always remains in the document then all mutations can
> be monitored by observers on the document (or document.body), but
> - if the fragment is removed from the document followed by mutation
> observers being called, then any further mutations won't be delivered to
> the observers, even when the fragment is reinserted into the document.
 > The exact behavior in this scenario depends on whether mutations
 > complete within one microtask or more than one

If the modifications to the fragment are done during the same microtask, 
then the observer will just get notified about those modifications. If 
in different microtask, then observer should observe
that fragment (so when the fragment is removed from document, 
observer.observe(root_of_fragement, options) should be called.).

If there was just a global - per document observers, those wouldn't
handle all the cases when node is adopted to and from other documents.
Also, such observers would make all the DOM mutations slower, since
the callback would need to be called all the time.

The proposed API allows one to restrict mutation observing to certain 
set of nodes. Mutations outside that set can be kept as fast as having 
no mutationobservers at all.
And also, since the observed set can expand, and isn't limited to same 
document handling, it can easily handle cases when nodes are moved to 
some other document.


> Sean.
Received on Wednesday, 12 October 2011 13:35:02 UTC

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